1. Field of the Invention
The present invention relates to a rotational speed differential responsive type torque transmitting assembly, and also to a differential unit incorporating a rotational speed differential responsive type torque transmitting assembly.
2. Description of the Prior Art
A differential unit with a rotational speed differential responsive type torque transmitting assembly is known from Japanese Patent Provisional Publication No. 61-62642.
This differential unit comprises a rotary casing, and a pinion assembly rotatable with the casing. The pinion assembly includes a pinion carrier and pinions on the pinion carrier. A pair of side gears are in mesh with the pinions. Rotatable with these side gears are a pair of rotary cams, respectively. These rotary cams are arranged in a face-to-face spaced relationship, and have a sleeve disposed therebetween. A partition within the sleeve divides the interior thereof into first and second cylindrical chambers. The cylindrical chambers open toward the cam surfaces of the rotary cams, respectively. A body of silicon oil fills these chambers. A pair of plungers are received in the chambers and becomes slidably engaged with the cam surfaces of the rotary cams, respectively. The partition is formed with an orifice which allows a restricted fluid flow communication between the chambers.
This differential unit has numerous problems. When a rotational speed differential occurs between the pair of side gears, the plungers are caused to reciprocate therefore subjecting the oil within the sleeve to adiabatic compression. This adiabatic compression causes the oil temperature to increase in volume. In this instance, since the sleeve is arranged eccentric with the rotation of the rotary casing, the oil within the sleeve is subjected to a centrifugal force, resulting in a further increase in volume of oil. As a result, a substantially great internal force builds up, causing a deformation of oil rounded tops of the plungers and damage on the cam surfaces.
Such a great internal force will inevitably cause a breakage of the seal, and therefore a leakage of oil, which leads to a shortage of oil after a long use.
Upon an occurence of such a rotational speed differential, the plungers urge the casing and its cover apart from each other. Since the rotary casing is rotatably supported by side bearings, these bearings are subject to this stress from the casing. As a result, the side bearings become bulky and need increased installation space.
With a view to solving the above problems, a rotational speed differential responsive type torque transmitting assembly for a differential unit has been proposed as disclosed in U.S. patent application Ser. No. 07/267,602 filed on Nov. 7, 1988 and assigned to the same assignee of this application. In this torque transmitting assembly, orifices of a fixed or constant opening are employed for providing restricted fluid communication between associated pressure chambers and an accumulator chamber.
This assembly provides a torque transmitting characteristic represented by, for example, a solid line curve "A" or "B" in the graph of FIG. 4, i.e., a curve of secondary order represented by y=a x.sup.2 where "y" is a differential slip limiting torque, "x" is a rotational speed differential and "a" is a constant.
In the case where the assembly is designed to have the torque transmitting characteristic "A" and the driver is skilled in a sport driving technique, he/she can drive through a corner rapidly by accelerating the vehicle during cornering and thereby lifting the inside driving wheel above the ground so that a large torque is transmitted to the outside driving wheel and at the same time the side forces on the tires are reduced.
When, however, the driver is not skilled in such a sport driving technique and he/she happens to depress the accelerator pedal too much during cornering, the torque transmitted to the outside driving wheel becomes too large, and the side forces on the tires are reduced considerably, thus putting the vehicle in a condition of being liable to spin.
In the case where the assembly is designed to have the characteristic "B", it cannot effect a desired differential slip limiting action when the vehicle travels on a split-u road surface. This is because the assembly can produce a sufficiently large differential slip limiting torque only when the rotational speed differential becomes considerably large. Furthermore, the assembly cannot prevent tacking-in and cannot improve the braking ability during cornering to a desired extent.
It is also known to utilize a viscous coupling instead of a torque transmitting assembly of the above described kind. In this instance, a torque transmitting characteristic represented by, for example, a one-dot chain line curve "C" in the graph of FIG. 4 is obtained. As will be apparent from this graph, a desiredly large differential slip limiting torque cannot be obtained when the roational speed differential is large. When, on the contrary, the viscous coupling is designed to have a torque transmitting chracteristic represented by, for example, the one-dot chain line curve "D" in FIG. 4, i.e., designed so as to produce a desiredly large differential slip limiting torque when the rotational speed differential is large, it produces, even when the rotational speed differential is small, an undesiredly large differential slip limiting torque which will cause understeer.